ASoC: spdif_transmitter: Allow 192kHz sample rate

Transmitters and receivers may support a 192kHz sample rate.

Tested with a Cubox-i imx6 system and an Onkyo TX-SR607 receiver.

Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
Signed-off-by: Mark Brown <broonie@linaro.org>